Semiconductor display device

ABSTRACT

It is an object of the present invention to provide a semiconductor display device using a protective circuit in which dielectric breakdown is prevented more effectively. In the invention, in the cases that a first interlayer insulating film is formed covering a TFT used for a protective circuit and a second interlayer insulating film, which is an insulating coating film, is formed covering a wiring formed over the first interlayer insulating film, a wiring for connecting the TFT to other semiconductor elements is formed so as to be in contact with the surface of the second interlayer insulating film so as to secure a path discharging charge accumulated in the surface of the second interlayer insulating film. Note that the TFT used for the protective diode is a so-called diode-connected TFT in which either of the first terminal or the second terminal is connected to a gate electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor display device using aprotective circuit which can prevent a dielectric breakdown of a lightemitting element from occurring.

2. Description of the Related art

How to restrain degradation of the light emitting element or chargingphenomena (charging) which leads to dielectric breakdown is an importantsubject in a manufacturing step of a semiconductor device. Specifically,with high integration, there is a tendency of not only miniaturizationof a channel length but also decrease in film thickness of variousinsulating films typified by a gate insulating film. Consequently, thedielectric breakdown due to the charging becomes a more serious problem.

The cause and environment to occur the charging is extremely complicatedand vary. Therefore, it is necessary not only to investigate the causeand the environment to occur the charge, but also to take measures in astructure of the semiconductor device so as to enhance resistance to thedegradation or dielectric breakdown due to the charging. In order toprevent the degradation or dielectric breakdown due to the charge, it iseffective to ensure a discharge path by the protective circuit using adiode (protective diode). By ensuring the discharge path, chargeaccumulated in an insulating film is prevented from being discharged inthe vicinity of the semiconductor element. Thus, phenomena (ESD:Electro-Static Discharge) in which the semiconductor element isdeteriorated and damaged due to energy of discharge can be prevented.

In addition, by providing a protective circuit, even if noise isinputted into a wiring along with a signal and a power supply voltage, asubsequent circuit can be prevented from malfunctioning due to thenoise, and a semiconductor device can be prevented from beingdeteriorated or a damaged due to the noise.

In a semiconductor display device typified by a liquid crystal displaydevice and a light emitting device, an active matrix type in whichsupply of signal to a display element can be maintained to some extentafter inputting a video signal can respond flexibly to a panel with abig size and high precision, and thus the semiconductor display devicewith an active matrix type has become future mainstream. A structure ofa pixel in an active matrix type semiconductor display device which isproposed concretely depends on each of manufacturer in a measure.Usually, a display element such as a light emitting element and a liquidcrystal element, and a thin film transistor (hereinafter referred to asTFT) for controlling operation of the display element are provided foreach of the pixels.

In the semiconductor display device, there are two kinds of structuresthat a display element is formed with a wiring connected directly to afirst or a second terminal of a TFT over an insulating film (a firstinterlayer insulating film) which covers the TFT, and that a displayelement is formed over an insulating film (a second interlayerinsulating film) which covers the wiring moreover. In the case of alight emitting device where light from the light emitting element istaken out from the side opposite to the TFT, the latter structure ismore desirable for being able to increase contrast more than the formerstructure since ratio of a region which contributes to luminescence inthe entire pixel portion can be increased.

The second interlayer insulating film covering the wiring moreover has apossibility of affecting characteristic of the display element due toits surface unevenness, and therefore, is formed by using a coatingmethod which is easy to flatten. This insulating coating film has aproblem of being easily charged when it is formed. However, a TFT isusually used for a protective diode, and the TFT is formed at the samelayer as TFTs constituting other circuits. Accordingly, the charge whichis charged in an insulating film in a layer formed over an insulatingfilm which covers the TFT used for the protective diode is hard todischarge through a discharge path secured by the protective circuit.Consequently, there has been a problem that dielectric breakdown easilyoccurs due to the charge.

BRIEF SUMMARY OF THE INVENTION

In view of the above problems, it is an object of the present inventionto provide a semiconductor display device using a protective circuitwhich can effectively prevent dielectric breakdown.

In the present invention, in the case where a first interlayerinsulating film is formed to cover a TFT used for a protective diode anda second interlayer insulating film, which is an insulative coatingfilm, is formed to cover a wiring formed over the first interlayerinsulating film, a wiring for connecting the TFT to other semiconductorelements is formed to being in contact with the surface of the secondinterlayer insulating film in order to secure a path for discharging thecharge accumulated in the surface of the second interlayer insulatingfilm. Note that the TFT used for the protective diode is so-called adiode-connected TFT in which either of a first terminal or a secondterminal of the TFT is connected to a gate electrode.

In this specification, either of the first terminal or the secondterminal corresponds to a source region, and the other corresponds to adrain region. The source and the drain regions depend on the electricpotential relation between the first terminal and the second terminal.Specifically, in the case of an n-channel type TFT, in the first and thesecond terminals, the terminal with lower electric potential correspondsto a source region and the terminal with higher electric potentialcorresponds to a drain region. Further, in the case of a p-channel typeTFT, in the first and the second terminals, the terminal with higherelectric potential corresponds to a source region and the terminal withlower electric potential corresponds to a drain region.

In the n-channel type TFT which is diode-connected, the first terminaland the gate electrode are presumed to be connected. In this case, whenthe electric potential of the first terminal is higher than that of thesecond terminal, the first terminal corresponds to a drain region andthe second terminal corresponds to a source region, respectively. Thenthe n-channel type TFT is turned on. Accordingly, a forward current fromthe first terminal to the second terminal can be obtained. To thecontrary, when the electric potential of the first terminal is lowerthan that of the second terminal, the first terminal corresponds to asource region and the second terminal corresponds to a drain region,respectively. Then the n-channel type TFT is turned off.

Further, in the p-channel type TFT which is diode-connected, the firstterminal and the gate electrode are presumed to be connected. In thiscase, when the electric potential of the first terminal is higher thanthat of the second terminal, the first terminal and the second terminalcorrespond to a source region and a drain region, respectively. Then thep-channel type TFT is turned off. To the contrary, when the electricpotential of the first terminal is lower than that of the secondterminal, the first terminal and the second terminal correspond to adrain region and a source region, respectively. Then the p-channel typeTFT is turned on. Accordingly, a forward current from the secondterminal to the first terminal can be obtained.

Specifically, in a semiconductor display device of the presentinvention, one feature of the invention is that a TFT used for aprotective diode is formed over an insulating surface, a firstinterlayer insulating film is formed to cover the TFT, a secondinterlayer insulating film is formed to cover the first interlayerinsulating film, either a first or a second terminal of the TFT isconnected to a gate electrode with a first wiring, the first wiring isconnected to a second wiring, the first wiring is formed to be incontact with the surface of the first interlayer insulating film, andthe second wiring is formed to be in contact with the surface of thesecond interlayer insulating film.

In a semiconductor display device of the invention, one feature of theinvention is that: a first TFT for supplying a display element with asignal, and a second TFT and a third TFT used for a protective diode areformed over the insulating surface; a first interlayer insulating filmis formed to cover the first TFT, the second TFT, and the third TFT; asecond interlayer insulating film is formed to cover the firstinterlayer insulating film; a display element is formed over the secondinterlayer insulating film; a first terminal or a second terminal of thefirst TFT is connected to the display element by a first wiring or asecond wiring; a first terminal of the second TFT and a gate electrodeare connected by a third wiring; a first terminal or a second terminalof the third TFT is connected to the gate electrode by a fourth wiring;a second terminal of the second TFT is connected to the fourth wiring bya fifth wiring and a sixth wiring; the first wiring, the third wiring,the fourth wiring, and the fifth wiring are formed so as to be incontact with the surface of the first interlayer insulating film; andthe second wiring and the sixth wiring are formed so as to be in contactwith the surface of the second interlayer insulating film.

In addition, one feature of another structure of a semiconductor displaydevice of the invention is that: a first TFT for supplying a displayelement with a signal, a second TFT and a third TFT used for aprotective diode are formed over an insulating surface; a firstinterlayer insulating film is formed to cover the first TFT, the secondTFT, and the third TFT; a second interlayer insulating film is formed tocover the first interlayer insulating film; a display element is formedover the second interlayer insulating film; a first terminal or a secondterminal of the first TFT is connected with the display element by afirst wiring and a second wiring; a first terminal of the second TFT anda gate electrode are connected with a third wiring; a second terminal ofthe second TFT, a first terminal or a second terminal of the third TFT,and a gate electrode are connected with a fourth wiring; the fourthwiring is connected with a fifth wiring; the first wiring, the thirdwiring, and the fourth wiring are formed so as to be in contact with thesurface of the first interlayer insulating film; and the second wiringand the fifth wiring are formed so as to be in contact with the surfaceof the second interlayer insulating film.

In the present invention, according to the above mentioned structure, inthe case where a first interlayer insulating film is formed to cover twoTFTs used for a protective diode and a second interlayer insulating filmis formed to cover a wiring formed over the first interlayer insulatingfilm, a path for discharging charge accumulated in the surface of thesecond interlayer insulating film can be, secured. Thus, a phenomenon todamage a semiconductor element can be prevented by discharging thecharge in the surface of the second interlayer insulating film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C show a circuit diagram and cross sectional views of aprotective circuit of the present invention.

FIGS. 2A and 2B are circuit diagrams showing a protective circuit of theinvention.

FIG. 3 is a top view of a substrate provided with a protective circuitof the invention.

FIGS. 4A to 4E are circuit diagrams of a protective circuit of theinvention.

FIGS. 5A to 5C are views showing a method for manufacturing asemiconductor display device of the invention.

FIGS. 6A and 6B are views showing a method for manufacturing asemiconductor display device of the invention.

FIGS. 7A and 7B are views showing a method for manufacturing asemiconductor display device of the invention.

FIGS. 8A and 8B are diagrams showing positional relationship of a signalline driver circuit and a protective circuit included in a semiconductordisplay device of the invention.

FIG. 9 is an equivalent circuit schematic of a signal line drivercircuit and a protective circuit included in a semiconductor displaydevice of the invention.

FIG. 10 is an equivalent circuit schematic of a scanning line drivercircuit and a protective circuit included in a semiconductor displaydevice of the invention.

FIGS. 11A to 11C are equivalent circuit schematics of a pixel includedin a light emitting device of the invention.

FIGS. 12A and 12B are equivalent circuit schematics of a pixel includedin a light emitting device of the invention.

FIG. 13 is a top view of a pixel included in a light emitting device ofthe invention.

FIGS. 14A and 14B are a top view and a cross sectional view of a lightemitting device of the invention.

FIGS. 15A to 15C are diagrams of electric equipment using asemiconductor display device of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A structure of a protective circuit used for a semiconductor displaydevice of the present invention is described with reference to FIGS. 1Ato 1C. In the protective circuit of the invention, a protective diode isprovided for a discharge path. FIG. 1A corresponds to a circuit diagramshowing a mode of the protective circuit of the invention. And at leastone of diode-connected TFTs 101 and 102 are provided for the path whichdischarges positive charge and the path which discharges negative chargerespectively.

Concretely, in each of the TFTs 101 and 102, a first terminal and a gateelectrode are connected. And the electric potential Vdd is given to thefirst terminal in the TFT 101, and the electric potential Vss is givento a second terminal in the TFT 102. In this specification, it isassumed that electric potential Vss is less than electric potential Vdd.Further a second terminal of the TFT 101 and the first terminal of theTFT 102 are electrically connected each other, and further electricallyconnected to the semiconductor element to be protected each other. It isassumed that the second terminal of the TFT 101 and the first terminalof the TFT 102 are electrically connected to a wiring 103.

In addition, in FIG. 1A, an example that both of the TFTs 101 and 102are p-channel type TFTs are shown; however, either of the TFTs 101 or102 may be an n-channel type TFT or both of the TFTs 101 and 102 may ben-channel type TFTs. In the case of using the n-channel type TFT, thefirst terminal and the gate electrode are connected.

And in the protective circuit of the invention, wirings for electricallyconnecting the TFTs used for the protective diode, specifically; thelayout thereof has a characteristic. FIG. 1B shows a cross sectionalview of the TFT 101 and the TFT 102 as an example.

The TFT 101 includes an island shape semiconductor film 104, a gateinsulating film 105 being in contact with the island shape semiconductorfilm 104, and a gate electrode 106 overlapped with the island shapesemiconductor 104 while sandwiching the gate insulating film 105therebetween. The island shape semiconductor film 104 includes a channelformation region 107 overlapped with the gate electrode 106, and alsoincludes a first terminal 108 and a second terminal 109 corresponding toa source or drain region sandwiching the channel formation region 107therebetween.

The TFT 102 includes an island shape semiconductor film 114, a gateinsulating film 105 being in contact with the island shape semiconductorfilm 114, and a gate electrode 116 overlapped with the island shapesemiconductor film 114 while sandwiching the gate insulating film 105therebetween. The island shape semiconductor film 114 includes a channelformation region 117 overlapped with the gate electrode 116, and alsoincludes a first terminal 118 and a second terminal 119 which correspondto a source region or a drain region sandwiching the channel formationregion 117 therebetween.

And the TFTs 101 and 102 are covered with a first interlayer insulatingfilm 120 having a single or a plurality of insulating films. And,wirings 121 to 124 connected to the TFTs 101 and 102 through contactholes formed in the first interlayer insulating film 120 are formed soas to be in contact with the surface of the first interlayer insulatingfilm 120.

Specifically, the wiring 121 is connected to the first terminal 108 andthe gate electrode 106 of the TFT 101, and the wiring 122 is connectedto the second terminal 109 of the TFT 101. In addition, the wiring 123is connected to the first terminal 118 and the gate electrode 116 of theTFT 102, and the wiring 124 is connected to the second terminal 119 ofthe TFT 102.

In FIG. 1B, each of the wirings 121 to 124 is formed from a singlewiring, however, the invention is not limited thereto. The wirings 121to 124 may be formed from a plurality of wirings each of which iselectrically connected each other.

A second interlayer insulating film 125 is formed over the firstinterlayer insulating film 120 so as to cover the wirings 121 to 124.Since a display element is required to be formed over the secondinterlayer insulating film 125, the surface thereof is desirably wellflattened, and preferably formed by a coating method. Note that thesecond interlayer insulating film 125 may be formed of a singleinsulating film or a plurality of insulating films. In each case, atleast a layer of the insulating films is formed by the coating methodpreferably.

And a wiring 126 connected to the wirings 122 and 123 through contactholes formed in the second interlayer insulating film 125 is formed soas to be in contact with the surface of the second interlayer insulatingfilm 125. By using the wirings 122, 123, and 126, the second terminal109 of the TFT 101, and the first terminal and the gate electrode 116 ofthe TFT 102 are electrically connected. And, the wirings 122, 123, and126 are electrically connected to a wiring 103 shown in FIG. 1A.

In FIG. 1B, the wiring 126 is constituted from a single wiring; however,the invention is not limited to this structure. The wiring 126 may beconstituted from a plurality of wirings each of which is connectedelectrically.

In addition, the wirings 122 and 123 are electrically connected throughthe wiring 126 in the invention; however, the invention is not limitedto this structure. For example, as shown in FIG. 1C, the second terminal109 of the TFT 101, and the first terminal 118 and the gate electrode116 of the TFT 102 may be connected by a wiring 127 formed so as to bein contact with the surface of the first interlayer insulating film 120.The wiring 127 may be constituted of a single wiring or a plurality ofwirings in which each of wirings is electrically connected. And thewiring 127 is also connected to the wiring 126.

In FIG. 1B, the wirings which are directly connected to the TFTs can beshortened in comparison to those in FIG. 1C, thereby preventingdielectric breakdown from occurring in the TFTs 101 and 102 by anantenna effect. Further, in FIG. 1C, a path for discharging is alreadysecured by the protective circuit before forming the second interlayerinsulating film 125; ESD due to charging can be certainly prevented.

And in FIG. 1B and FIG. 1C, for example, in the case where positivecharge is charged on the surface of the second interlayer insulatingfilm 125 and electric potential Vdd′ having higher electric potentialthan electric potential Vdd is given to the wiring 126, the TFT 101 isturned on and the TFT 102 is turned off as shown in FIG. 2A. Accordinglythe positive charge is discharged through the TFT 101. In addition, inFIG. 1B and FIG. 1C, in the case where negative charge is charged on thesurface of the second interlayer insulating film 125 and electricpotential Vss′ having lower electric potential than electric potentialVss is given to the wiring 126, the TFT 101 is turned off and the TFT102 is turned on as shown in FIG. 2B. Accordingly the negative charge isdischarged through the TFT 102.

Accordingly, in each case, the electric potential that is higher thanthe electric potential Vdd and lower than the electric potential Vss isnot given to the wiring 103. Therefore, damage due to charging to asemiconductor element electrically connected to the wiring 103 can beprevented.

Note that the protective circuit is specifically effective in preventingthe semiconductor element from being deteriorated and damaged due to thecharging caused when the second interlayer insulating film is formed bya coating method. However, in the semiconductor display device of theinvention, the second interlayer insulating film is not limited to acoating film. The cause and environment to occur charging are extremelycomplicated and vary, and therefore it is undeniable that charging mayoccur when the second interlayer insulating film is not formed by thecoating method. Consequently, the invention is effective for preventingESD in the case the second interlayer insulating film is formed by amethod other than the coating method, for example, by vapor deposition,sputtering, or chemical vapor deposition (CVD).

Next, a structure of a semiconductor display device of the invention isdescribed with reference to FIG. 3. FIG. 3 is a top view of a substrate130 included in the semiconductor display device. A pixel portion 131, ascanning line driver circuit 132 for selecting a pixel included in thepixel portion 131, and a signal line driver circuit 133 for supplying avideo signal for the selected pixel are formed over the substrate 130.Further, a reference numeral 134 corresponds to an input terminal forsupplying a signal or a power supply potential into the each circuitformed over the substrate 130.

And reference numerals 135 to 137 correspond to protective circuits. Thevarious circuits formed over the substrate 130 are connected each otherwith wirings and the wirings are connected to the protective circuits135 to 137.

Specifically, the input terminal 134 and the signal line driver circuit133 are connected by a wiring 140, and the protective circuit 135 isconnected to the wiring 140. Each of the semiconductor elements includedin the signal line driver circuit 133 is protected by the protectivecircuit 135.

The signal line driver circuit 133 and the pixel portion 131 areconnected by a signal line 141, and the protective circuit 136 isconnected to the signal line 141. Each of the semiconductor elementsincluded in the signal line driver circuit 133 and the pixel portion 131is protected by the protective circuit 136. Note that the protectivecircuit 136 has at least a connection with the signal line 141.Therefore, the protective circuit 136 may be provided between the signalline driver circuit 133 and the pixel portion 131, or may be provided onthe other side of the signal line driver circuit 133 sandwiching thepixel portion 131 therebetween as shown in FIG. 3. Though not shown infigure, the protective circuit 136 may be provided between the signalline driver circuit 133 and the input terminal 134.

Further, the scanning line driver circuit 132 and the pixel portion 131are connected by a scanning line 142, and the protective circuit 137 isconnected to the scanning line 142. With the use of the protectivecircuit 137, each of the semiconductor elements included in the scanningline driver circuit 132 and the pixel portion 131 can be protected. Notethat the protective circuit 137 has at least a connection with thescanning line 142. The protective circuit 137 may be provided betweenthe scanning line driver circuit 132 and the pixel portion 131, or maybe provided on the other side of the scanning line driver circuit 132sandwiching the pixel portion 131 therebetween as shown in FIG. 3.Though not shown in figure, the protective circuit 137 may be providedbetween the scanning line driver circuit 132 and the input terminal 134.

Note that not all of the protective circuits 135 to 137 are required tobe provided, one of them or a plurality of protective circuits amongthem may be provided.

In this invention, the protective circuit serves not only to dischargecharge in the second interlayer insulating film, but also to decreasenoise inputted to the wiring along with the signal or the power supplyvoltage. As a result, a situation that the semiconductor element isdeteriorated or damaged due to the noise can be prevented.

In FIG. 3, the signal line driver circuit 133 and the scanning linedriver circuit 132 are formed over the substrate 130 in which the pixelportion 131 is formed. However, the invention is not limited to thisstructure. For example, in the case of using an amorphous semiconductoror a microcrystal semiconductor as the semiconductor elementconstituting the pixel portion 131, the signal line driver circuit 133and the scanning line driver circuit 132 which are formed separately maybe mounted over the substrate 130 by a known method such as a COG methodor a TAB method. In this case, the protective circuit is connected tothe wiring which connects the input terminal and the pixel portion.Further, in the case of using the microcrystal semiconductor as theelement constituting the pixel portion 131, the scanning line drivercircuit and the pixel portion may be formed of the microcrystalsemiconductor on the same substrate, and then the signal line drivercircuit may be equipped with the substrate. And one part of the scanningline driver circuits or one part of the signal line driver circuits maybe formed over the same substrate along with the pixel portion, and thenother parts of the scanning line driver circuits or other part of thesignal line driver circuits may be equipped with the substrate. Namely,because there are a variety of modes in the protective circuit, thenumber of and the location of the protective circuit are to be fixed inaccordance with the mode.

Next, a specific example of the protective circuit used in thisinvention is described with reference to FIGS. 4A to 4E.

The protective circuit shown in FIG. 4A includes protective diodes 401to 404 using a plurality of TFTs. The protective diode 401 includes twop-channel type TFTs 401 a and 401 b which are serially connected. Andone end of the p-channel type TFTs 401 a and 401 b serially connected isconnected to a gate electrode of the two p-channel type TFTs 401 a and401 b. Other protective diodes 402 to 404, as with the protective diode401, also include a plurality of TFTs in which each of TFTs is seriallyconnected. And one end of the plurality of TFTs serially connected isconnected to the gate electrode of the plurality of TFTs.

In the invention, the number and the polarity of TFTs in each of theprotective diodes 401 to 404 are not limited to the structure shown inFIG. 4A.

Protective diodes 401 to 404 are serially connected in order, and a nodebetween the protective diode 402 and the protective diode 403 isconnected to a wiring 405. The wiring 405 is presumed to be connected toa semiconductor element which becomes a subject of protection. The nodeconnecting with the wiring 405 is not limited to the node between theprotective diode 402 and the protective diode 403. And the nodeconnecting to the wiring 405 may be any one of the plurality of nodesamong the protective diodes 401 to 404 serially connected.

Electric potential Vss is given to an end of the protective diodes 401to 404 serially connected, and electric potential Vdd is given to theother end. Further, each of the protective diodes 401 to 404 isconnected in a direction that voltage of reverse bias is applied.

A protective circuit shown in FIG. 4B includes protective diodes 410 and411, capacitor elements 412 and 413, and a resistance element 414. Theresistance element 414 is a resistance of two terminals, and oneterminal is given electric potential Vin that is to be applied to awiring 415, and the other is given electric potential Vss. Theresistance element 414 is provided for shifting the electric potentialof the wiring 415 to the electric potential Vss when the electricpotential Vin become impossible to be applied, and the resistance valueis set to be much larger than wiring resistance of the wiring 415. Ap-channel type TFT which is diode-connected is used for each of theprotective diodes 410 and 411.

When the electric potential Vin is higher than the electric potentialVdd, according to the voltage between the gate electrode and the sourceregion, the p-channel type TFT included in the protective diode 410 isturned on, and the p-channel type TFT included in the protective diode411 is turned off. Thus, the electric potential Vdd is given to thewiring 415 through the protective diode 410. Therefore, even when theelectric potential Vin becomes higher than the electric potential Vdddue to noise or the like, the electric potential to be given to thewiring 415 does not become higher than the electric potential Vdd. Onthe other hand, when the electric potential Vin is lower than theelectric potential Vss, according to the voltage between the gateelectrode and the source region, the p-channel type TFT included in theprotective diode 410 is turned off and the p-channel type TFT includedin the protective diode 411 is turned on. Accordingly, the electricpotential Vss is given to the wiring. Therefore, when the electricpotential Vin becomes lower than the electric potential Vss due to noiseor the like, the electric potential to be given to the wiring 415 doesnot become lower than the electric potential Vss. Furthermore, by theuse of the capacitor elements 412 and 413, pulsed noise of inputtedelectric potential Vin can be decreased and sudden change of theelectric potential due to the noise can be decreased to some extent.

By the layout of the protective circuit as above mentioned, the electricpotential of the wiring is kept between the electric potential Vss andVdd, and it becomes possible to prevent the extremely high or lowvoltage out of this electric potential from being applied to thesubsequent circuit. Further, by providing the protective circuit to theinput terminal in which the signal is inputted, the electric potentialof all of the wirings in which the signal is applied can be keptconstant height (here, electric potential Vss). Namely, when the signalis not inputted, it also has a function of a short ring which can shortwirings. Therefore, dielectric breakdown caused by electric potentialdifference between wirings can be prevented. Further, when the signal isinputted, since the resistance value of the resistance element 414 ishigh enough, the signal applied to the wiring is not pulled by theelectric potential Vss.

FIG. 4C is an equivalent circuit schematic of a protective circuit inwhich the protective diodes 410 and 411 are substituted by two p-channeltype TFTs.

Although a p-channel type TFT which is diode-connected is used as theprotective diode in the protective circuits shown in FIGS. 4B and 4C,the invention is not limited to the structure. As the protective diode,an n-channel type TFT which is diode-connected may be used.

Protective diodes 420 to 427 and a resistance element 428 are includedin the protective circuits shown in FIG. 4D. The resistance element 428and a wiring 429 are serially connected. The n-channel type TFT which isdiode-connected is used for each of the protective diodes 420 to 423.And the p-channel type TFT each of which is diode-connected is used foreach of the protective diodes 424 to 427.

The protective diodes 420 and 421 are serially connected, and electricalpotential Vss is given to one end and the other end is connected to thewiring 429. The protective diodes 422 and 423 are serially connected,and electrical potential Vdd is given to one end and the other end isconnected to the wiring 429. The protective diodes 424 and 425 areserially connected, and electrical potential Vss is given to one end andthe other end is connected to the wiring 429. The protective diodes 426and 427 are serially connected, and electrical potential Vdd is given toone end and the other is connected to the wiring 429.

Further, a protective circuit shown in FIG. 4E includes resistanceelements 430 and 431 and a protective diode 432. In FIG. 4E, then-channel type TFT which is diode-connected is used as the protectivediode 432; however the invention is not limited to this structure.Alternatively, the p-channel type TFT which is diode-connected may beused or a plurality of TFTs each of which is diode-connected may beused. The resistance elements 430 and 431, and the protective circuit432 are serially connected to a wiring 433.

By using the resistance elements 430 and 431, sudden change of theelectric potential in the wiring 433 can be relaxed, and deteriorationor damage of the semiconductor element can be prevented. Further, theuse of the protective diode 432 can prevent reverse bias current fromflowing toward the wiring 433 due to the fluctuation of the electricpotential.

In the case of connecting only the resistance elements serially to thewiring, the sudden fluctuation of the electric potential in the wiringcan be relaxed, and deterioration or damage of the semiconductor elementcan be prevented. In addition, in the case of connecting only theprotective diode to the wiring, the flow of the reverse current towardthe wiring due to the fluctuation of the electric potential can beprevented.

Embodiment 1

Next, a specific method for manufacturing a light emitting device whichcorresponds to a mode of a semiconductor display device of the presentinvention is described. In this embodiment, an example of manufacturinga TFT used for the protective circuit and a TFT for controlling currentsupply to a light emitting element over the same substrate is described.

First, a base film 202 is formed on a substrate 201 as shown in FIG. 5A.A glass substrate such as barium borosilicate glass or aluminoborosilicate glass, a quartz substrate, a ceramic substrate, or the likecan be used for the substrate 201, for example. A metal substrateincluding a SUS substrate or a substrate in which an insulating film isformed over a surface of the silicon substrate may be used for thesubstrate 201. A substrate formed of a synthetic resin havingflexibility such as plastic or the like generally tends to have lowerresistance temperature compared to the above substrates, however asubstrate which can withstands the processing temperature in manufacturesteps can be used.

The base film 202 is formed in order to prevent an alkaline metal suchas Na and an alkaline earth metal contained within the substrate 201from diffusing into a semiconductor film and exerting an adverseinfluence on characteristics of a semiconductor element such as a TFT.The base film 202 is therefore formed by using an insulating film suchas a silicon oxide film, a silicon nitride film, or a silicon nitrideoxide film capable of suppressing the diffusion of the alkaline metaland the alkaline earth metal to the semiconductor film. In thisembodiment, the base film 202 is formed of a silicon nitride oxide tohave a thickness of 10 nm to 400 nm (preferably 50 nm to 300 nm) byplasma CVD.

Note that the base film 202 may be a single layer, or may be a laminateof a plurality of insulating films. Further, it is effective to form thebase film in order to prevent impurity diffusion in the case of using asubstrate that contains a certain amount of the alkaline metal or thealkaline earth metal, such as a glass substrate, or an SUS substrate.However, the base film need not to be formed in the case of using aquartz substrate or the like, in which impurity diffusion does notmatter.

Island shape semiconductor films 203 to 205 to be used as an activelayer are formed over the base film 202. The film thickness of theisland shape semiconductor films 203 to 205 are set from 25 nm to 100 nm(preferably from 30 nm to 60 nm). Note that the island shapesemiconductor film 203 to 205 may be an amorphous semiconductor, asemi-amorphous semiconductor (microcrystalline semiconductor), or apolycrystalline semiconductor. Further, not only silicon but alsosilicon germanium can be used for the semiconductor. It is preferablethat the germanium concentration is on the order of 0.01 to 4.5 atomic %when silicon germanium is used.

When a polycrystalline semiconductor is used, an amorphous semiconductoris formed first. The amorphous semiconductor may be then crystallized byusing a known method for crystallizing an amorphous semiconductor. Amethod for performing crystallization by heating using a heater, amethod for performing crystallization by laser light irradiation, amethod for performing crystallization by using a catalyst metal, amethod for performing crystallization by using infrared light, and thelike can be given as known methods of crystallization.

In the case of crystallizing the amorphous semiconductor film by usinglaser light, for example, a pulsed laser or a continuous wave excimerlaser, a pulsed laser or a continuous wave YAG laser, a pulsed laser ora continuous wave YVO₄ laser, or the like is used. For example, when aYAG laser is used, a wavelength of a second harmonic, which tends toeasily be absorbed by the semiconductor film, is desirably employed. Theoscillating frequency is set from 30 kHz to 300 kHz, the energy densityis set from 300 mJ/cm² to 600 mJ/cm² (typically from 350 mJ/cm² to 500mJ/cm²), and the scanning speed may be set so that several irradiationshots can be emitted at an arbitrary point.

Next, a TFT is formed by using the island shape semiconductor films 203to 205. In this embodiment, as shown in FIG. 5B, top gate type TFTs 206to 208 are formed by using the island shape semiconductor films 203 to205.

Specifically, a gate insulating film 209 is formed so as to cover theisland shape semiconductor films 203 to 205. A conductive film is formedand patterned over the gate insulating film 209, thereby forming gateelectrodes 210 to 212. And then, by using the gate electrodes 210 to 212or a resist as a mask and doping n-type or p-type impurities to theisland shape semiconductor films 203 to 205, a source region, a drainregion, and an LDD region and the like are formed. Here, the case whereall the TFTs 206 to 208 are p-type is described.

Silicon oxide, silicon nitride, silicon nitride oxide, or the like canbe used as the gate insulating film 209, for example. Further, plasmaCVD, sputtering, and the like can be used as the film formation method.For example, when the gate insulating film using silicon oxide isdeposited by using plasma CVD, film formation may be performed by usinga mixed gas of tetraethyl orthosilicate (TEOS) and O₂, at a reactionpressure of 40 Pa, a substrate temperature of 300° C. to 400° C., and aRF power density (13.56 MHz) of 0.5 W/cm²to 0.8 W/cm².

Further, aluminum nitride can also be used as the gate insulating film209. The thermal conductivity coefficient of aluminum nitride isrelatively high, and heat generated in a TFT can be diffusedefficiently. Further, a gate insulating film in which aluminum nitrideis laminated after forming silicon oxide, silicon oxynitride, or thelike, which contain no aluminum, may also be used.

According to the series of steps, TFTs 206 and 207 used for theprotective diode and a TFT 208 for controlling current to supply to alight emitting element can be formed. Note that the method formanufacturing the TFT is not limited to the above-mentioned steps. Agate electrode and a wiring may be manufactured by drop discharging.

Next, a passivation film 213 which corresponds to one part of a firstinterlayer insulating film is formed so as to cover the TFTs 206 to 208.An insulating film such as silicon oxide, silicon nitride, or siliconoxynitride each of which includes silicon is used for the passivationfilm 213, and the film thickness is set about from 100 nm to 200 nm.

Heat treatment is performed in order to activate an impurity elementadded to the island shape semiconductor films 203 to 205. This step canbe performed by thermal annealing using an annealing furnace, by laserannealing, or by rapid thermal annealing (RTA). For example, in the caseof performing activation by thermal annealing, it is performed at atemperature of 400° C. to 700° C. (preferably from 500° C. to 600° C.)under a nitrogen atmosphere containing an oxygen at its concentration ofequal to or less than 1 ppm, preferably equal to or less than 0.1 ppm.In addition, heat treatment is performed at 300° C. to 450° C. for 1 to12 hours within an atmosphere containing 3% to 100% hydrogen, therebyperforming hydrogenation of the island shape semiconductor films. Thisstep is performed for the purpose of terminating dangling bonds bythermally excited hydrogen. Plasma hydrogenation (using hydrogen excitedby plasma) may be also performed as the other means of hydrogenation.Further, the activation step may be performed before forming thepassivation film 213.

Next, as shown in FIG. 5C, a first insulating film 214 is formed so asto cover the passivation film 213. An insulating film including Si—Obonds and Si—CH_(x) bonds formed from an organic resin film, aninorganic insulating film, and a material of an siloxane group as astarting material can be used for the first insulating film 214. In thisembodiment, a lamination film of the first insulating film 214 and thepassivation film 213 corresponds to a first interlayer insulating film215. Note that the first interlayer insulating film 215 may be formed ofa single insulating film or a plurality of insulating films.

Then, the gate insulating film 209 and the first interlayer insulatingfilm 215 are etched to form contact holes. And then, wirings 216 to 221for connecting the island shape semiconductor films 203 to 205 and thegate electrodes 210 and 211 are formed so as to be in contact with thefirst interlayer insulating film 215.

In the TFT 206, a first terminal 223 which corresponds to a sourceregion or a drain region, and the gate electrode 210 are connected bythe wiring 216. And a second terminal 225 which corresponds to a sourceregion or a drain region of the TFT 206 is connected to the wiring 217.In the TFT 207, a first terminal 226 which corresponds to a sourceregion or a drain region is connected to the gate electrode 211 by thewiring 218. Further, a second terminal 228 which corresponds to a sourceregion or a drain region of the TFT 207 is connected to the wiring 219.In the TFT 208, a first terminal 229 which corresponds to a sourceregion or a drain region is connected to the wiring 220. A secondterminal 231 which corresponds to a source region or a drain region ofthe TFT 208 is connected to the wiring 221.

Next, as shown in FIG. 6A, a second insulating film 233 and a thirdinsulating film 234 are sequentially formed so as to cover the wirings216 to 221 and so as to be in contact with the surface of the firstinsulating film 214. The laminated film of the second insulating film233 and the third insulating film 234 corresponds to a second interlayerinsulating film 235. Note that the second interlayer insulating film 235may be formed of a single insulating film or a plurality of insulatingfilms.

An insulating film including Si—O bonds and Si—CH_(x) bonds formed froman organic resin film, an inorganic insulating film, and a material of asiloxane group as a starting material, can be used for the secondinsulating film 233. In this embodiment, the second insulating film 233is formed from an insulating film formed of a material of the siloxanegroup by a coating method. A film which is hardly penetrated by asubstance to promote deterioration of a light emitting element such asmoisture and oxygen, as compared with other insulating films, is usedfor the third insulating film 234. Typically, a silicon nitride filmformed by RF sputtering is used; however, a diamond like carbon (DLC)film, an aluminum nitride film, or the like is used.

Next, as shown in FIG. 6B, the second interlayer insulating film 235 isetched to form a contact hole. And, wirings 236 and 237 connected to thewirings 217, 218, and 221 are formed so as to be in contact with thesurface of the second interlayer insulating film 235. Specifically, thewiring 236 is connected to the wirings 217 and 218, and the wiring 237is connected to the wiring 221.

As shown in FIG. 7A, an anode 240 is formed so as to cover the wirings236 and 237, and to be in contact with the surface of the thirdinsulating film 234. The anode 240 is connected to the wiring 237. Asthe anode 240, a single layer film formed of one of TiN, ZrN, Ti, W, Ni,Pt, Cr, Ag, and the like or a plurality of those, a laminated filmmainly containing titanium nitride and aluminum, or a three layerstructure of a film mainly containing a titanium nitride film andaluminum and a titanium nitride film can be used. In this embodiment,TiN is used to form the anode 240.

In this embodiment, the case where light is taken out from a cathodeside is described, however, light may be taken out from the anode 240side. In this case, other light-transparent oxide conductive materialssuch as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide(IZO), and zinc oxide (GZO) to which gallium is added, can be used forthe anode 240. In addition, indium tin oxide including ITO and siliconoxide (hereinafter referred to as ITSO), or indium oxide includingsilicon oxide in which 2% to 20% of zinc oxide (ZnO) is further mixedmay be used.

Next, a barrier 241 is formed over the third insulating film 234. Aninsulating film including Si—O bonds and Si—CH_(x) bonds formed from anorganic resin film, inorganic insulating film, and a material of asiloxane group as a starting material can be used for the barrier 241.The barrier 241 is to cover an end portion of the anode 240, and toinclude an opening portion in the region overlapped with the anode 240.The end portion of the opening portion of the barrier 241 is desirablyrounded so as not to have a hole in the electroluminescent layer formedlater in the end portion. Specifically, it is preferable that a curvedcurvature radius which is drawn by a cross section of the barrier 241 inthe opening portion is approximately 0.2 μm to 2 μm.

In FIG. 7A, an example of using a positive type photosensitive acrylicresin as the barrier 241 is shown. In the photosensitive acrylic resin,there are a positive type in which a place exposed to an energy linesuch as light, electron, ion is removed and a negative type in which aplace exposed is left. In the invention, the negative type organic resinfilm may be used. Further, the barrier 241 may be formed of usingphotosensitive polyimide. In the case of forming the barrier 241 byusing the negative type acrylic, the end portion in the opening portionhas a sigmoid cross section. At this time the curvature radius in theupper end portion and bottom end portion in the opening portion isdesirably from 0.2 μm to 2 μm.

According to the above mentioned structure, coverage of anelectroluminescent layer and a cathode to be formed later can beimproved, and the anode 240 and the cathode can be prevented from beingshort-circuited in a hole formed in the electroluminescent layer.Further, by alleviating the stress of the electroluminescent layer, adefect called shrink in which a light emitting region is diminished canbe suppressed and the reliability can be thus improved.

In order that a surface of the anode 240 can be flattened, beforeforming the electroluminescent layer, the surface may be polished byCMP, polyvinyl alcohol porous body or the like.

Heat treatment in an atmospheric air or heat treatment in a vacuumatmosphere (vacuum bake) may be performed in order to remove absorbedmoisture, oxygen, or the like in the barrier 241 and the anode 240before forming the electroluminescent layer. Specifically, the heattreatment is performed in a vacuum atmosphere, with a substratetemperature of from 200° C. to 450° C., preferably 250° C. to 300° C.for about 0.5 to 20 hours. It is desirably set the vacuum atmosphere at3×10⁻⁷ Torr or less, and if possible at 3×10⁻⁸ Torr or less. In the casewhere the electroluminescent layer is formed after performing the heattreatment in the vacuum atmosphere, the reliability can be furtherimproved by setting the substrate in the vacuum atmosphere just beforeforming the electroluminescent layer. Also, the anode 240 may beirradiated with ultraviolet radiation before or after vacuum baking.

By forming an electrode which is formed so as to be in contact with thesecond interlayer insulating film 235 (in this embodiment, the anode240) from a light-transparent conductive oxide material such as ITSO anda conductive film including silicon oxide, and by forming an insulatingfilm being in contact with the electrode in the second interlayerinsulating film 235 (in this embodiment, the third insulating film 234)with silicon nitride, brightness of the light emitting element can beincreased than that of the combination in the case that the anode 240and the third insulating film 234 are formed of other materials. In thiscase, moisture is easily absorbed due to the silicon oxide included inthe anode 240, thus the above mentioned vacuum baking is extremelyeffective.

Next, as shown in FIG. 7B, an electroluminescent layer 242 is formedover the anode 240. The electroluminescent layer 242 is formed of asingle layer or a plurality of layers; each of the layers may include aninorganic material in addition to an organic material. In the case wherework function of the material used for the cathode is not adequatelysmall, the electroluminescent layer 242 is desirably provided with anelectron injection layer.

Next, a cathode 243 is formed to cover the electroluminescent layer 242.The anode 240, the electroluminescent layer 242, and the cathode 243 areoverlapped with each other in the opening portion of the barrier 241.And the overlapped portion corresponds to a light emitting element 244.

The cathode 243 has a light-transparent property. Specifically, otherlight-transparent conductive oxide materials such as indium tin oxide(ITO), zinc oxide (ZnO), indium zinc oxide (IZO), zinc oxide (GZO) towhich gallium is added can be used. Further, indium tin oxide includingITO and silicon oxide (hereinafter referred to as ITSO), indium oxideincluding silicon oxide in which 2 to 20% zinc oxide (ZnO) is furthermixed may be used. In this case, an electron injection layer ispreferably provided for the electroluminescent layer 242 so as to be incontact with the cathode 243.

Further, a metal with small work function, an alloy, an electricconductive compound, a compound of these material, or the like is formedwith a film thickness enough to transmit light, and can be used as ancathode 243. Specifically, in addition to an alkali metal such as Li andCs, and an alkaline earth metal such as Mg, Ca, and Sr, and an alloyincluding these (for example, Mg:Ag, Al:Li), a rare-earth metal such asYb or Er, or the like can be used to form the cathode 243 with a filmthickness of approximately 5 nm to 30 nm. When the electric injectionlayer is provided, another conductive layer such as Al is formed with afilm thickness enough to transmit light, and can be used as the cathode243. When the cathode 243 is formed with a film thickness enough totransmit light, a conductive layer having a light-transparent propertyis formed using a light-transparent conductive oxide material so as tobe in contact with the top or the bottom of the cathode 243, and thussheet resistance of the cathode may be suppressed. In the case wherelight is reflected from the cathode 243 side and preferably taken outonly from the anode 240 side, the cathode 243 may be formed with a filmthickness enough to reflect light.

After the light emitting element 244 is formed, a protective film may beformed over the cathode 243. As is the case with the third insulatingfilm 234, a film which is hardly penetrated by a substance promotingdeterioration of a light emitting element such as moisture and oxygen,as compared with other insulating films is used as the protective film.Typically, a DLC film, a carbon nitride film, a silicon nitride filmformed by RF sputtering, or the like is preferably used. Further, alaminated film of the film which is hardly penetrated by a substancesuch as moisture and oxygen and a film which is easily penetrated bymoisture and oxygen, as compared with the film, may be used as theprotective film.

In FIG. 7B, an example where the anode is formed closer to a substratethan the cathode is described, however the invention is not limited tothis structure. The cathode may be formed closer to the substrate thanthe anode.

After completing the step up to FIG. 7B in practice, packaging (sealing)is preferably performed by using a protective film (laminate film,ultraviolet curable resin film, or the like) or a light-transparentcover material with higher air tightness and a little degasification soas not to expose the light emitting element to the air. At this time, bymaking the inside of the cover material inert atmosphere, or byarranging a material having hygroscopic properties (for example, bariumoxide) in the inside, the reliability of the light emitting element isimproved.

Note that a method for manufacturing a light emitting device of theinvention is not necessarily limited to the above modes. Further, asemiconductor display device of the invention includes not only a lightemitting device but also a liquid crystal display device. The embodimentdescribed above specifically describes a mode of the invention.Therefore, the invention is not limited to the above mentionedembodiment, and changes and modifications according to the technicalidea of this invention are possible.

Further, a semiconductor display device may be formed by transferring asemiconductor element manufactured according to the above method to aflexible substrate such as plastic. There are various methods fortransferring the semiconductor element, for example, a method ofproviding a metal oxide film between the substrate and the semiconductorelement, separating the semiconductor element with embrittlement due tocrystallization of the metal oxide film and transferring; a method ofproviding an amorphous silicon film including hydrogen between thesubstrate and the semiconductor element, irradiating with laser light oretching to remove the amorphous silicon film, separating the substrateand the semiconductor element and transferring; a method of mechanicallyseparating the semiconductor element from the substrate in which thesemiconductor element is formed or separating the semiconductor elementfrom the substrate by etching with solution or gas and transferring orthe like. Note that the step of the transferring may be performed beforeor after manufacturing the display element.

Embodiment 2

In this embodiment, a positional relationship of a driver circuit and aprotective circuit included in a semiconductor display device of thepresent invention is described.

A block diagram of a signal line driver circuit of this embodiment isshown in FIG. 8A. In FIG. 8A, reference numeral 900 corresponds to apixel portion; 901, a signal line driver circuit; 902, a protectivecircuit; and 903, an input terminal. The signal line driver circuit 901includes a shift register 904, a latch A 905, and a latch B 906.

A clock signal (CLK) and a start pulse signal (SP) are inputted into theshift register 904. A conversion signal (L/R) may be inputted inaddition to the CLK and the SP. A timing signal is generated in theshift register 904 by inputting the CLK and the SP. The order ofoccurrence of pulse in the timing signal can be changed by L/R. Thegenerated timing signal is inputted into the latch A 905 of the firststage sequentially. When the timing signal is inputted into the latch A905, a video signal (VS) is written and stored into the latch A 905 inorder in synchronization with the pulse of the timing signal. In thisembodiment, the video signal is written into the latch A 905, howeverthe invention is not limited to this structure. The latch A 905constituted by a plurality of stages may be divided into some groups andthe video signals may be inputted to each of the groups in parallel.Namely, so-called division driving may be carried out. Note that thenumber of the groups at this time is referred to as a division number.For example, in the case of dividing the latch into groups by fourstages, it is referred to as a division driving in four.

A time required to terminate inputting video signals into all thelatches of the stages in the latch A 905 is referred to as a lineperiod. In practice, the line period may include a horizontal flybackperiod added to the aforementioned line period.

After a line period is terminated, a latch signal is supplied to thelatch B 906 of the second stage and the video signals stored in thelatch A 905 in synchronization with the latch signals is written intothe latch B 906 all at once and stored therein. Subsequent video signalsare inputted in synchronization with timing signals from the shiftregister 904 again into the latch A 905 after supplying the videosignals to the latch B 906. In one line period of this second sequence,the video signals supplied and stored in the latch B 906 are inputted tothe pixel portion 900 through a signal line 907.

Note that another circuit which can select a signal line such as adecoder circuit may be used as a substitute for the shift register 904.

In FIG. 8A, the protective circuit 902 is provided between the signalline driver circuit 901 and the pixel portion 900. The protectivecircuit 902 is connected to the signal line 907. By using the protectivecircuit 902, each of the semiconductor elements connected to the signalline 907 can be prevented from ESD.

In FIG. 8A, an example of connecting the protective circuit 902 to thesignal line 907 is shown, however, the protective circuit may beprovided for the wiring to input a video signal (VS) to the signal linedriver circuit 901.

FIG. 9 shows an example of a specific circuit diagram of the shiftregister 904, the latch A 905, the latch B 906 and the protectivecircuit 902 which are shown in FIG.8A. In FIG. 9, an example in which aprotective circuit 908 is also provided for the wiring to input thevideo signal (VS) is shown. As shown in FIG. 9, a buffer, an inverterand the like may be provided in the interval of circuits.

In FIG. 10, an example of a specific circuit diagram of a shift registerincluded in a scanning line driver circuit and a protective circuitconnected to the scanning line is shown. In FIG. 10, the scanning linedriver circuit includes a shift register 1101 and a buffer 1103.Reference numeral 1102 corresponds to a protective circuit and 1104corresponds to a scanning line. A level shifter may be included in thescanning line driver circuit. A select signal is generated in thescanning line driver circuit when the CLK and the SP are inputted to theshift register 1101. The generated select signal is thenbuffer-amplified in the buffer 1103 and then inputted to thecorresponding scanning line 1104. Gates of transistors in one line ofpixels are connected to the scanning line 1104. As the transistors ofone line of pixels have to be turned ON simultaneously, the buffer 1103is required to be capable of applying a large current. The protectivecircuit 1102 is connected to the scanning line 1104.

Note that another circuit such as a decoder circuit that can select asignal line may be used instead of the shift register 1101.

In FIG. 8A, the protective circuit 902 connected to the signal line 907is provided between the signal line driver circuit 901 and the pixelportion 900, however the invention is not limited to this structure. Theposition of the protective circuit 902 can be freely changed by formingthe signal line 907 to be in contact with the surface of the firstinterlayer insulating film, connecting the signal line 907 to the wiringformed so as to be in contact with the surface of the second interlayerinsulating film, and extending.

In FIG. 8B, an example that a protective circuit 902 connected to thesignal line 907 is provided between the input terminal 903 and thesignal line driver circuit 901 is shown. By extending a wiring 909, asealant 910 for sealing a light emitting element between a substrate anda cover material and the protective circuit 902 can be overlapped, thusspaces can be effectively used.

Note that the signal line driver circuit and the scanning line drivercircuit included in the semiconductor display device of the inventionare not limited to the above structure. A designer can freely design thenumber of the signal line driver circuits and the layout thereof.

Embodiment 3

Next, a pixel of a light emitting device which corresponds to a mode ofa semiconductor display device of the present invention is describedwith reference to FIGS. 11A to 11C. FIG. 11A shows an equivalent circuitschematic of a pixel, which includes a signal line 6114, power supplylines 6115 and 6117, a scanning line 6116, a light emitting element6113, a TFT 6110 for controlling input of a video signal to the pixel, aTFT 6111 for controlling current value applied to both electrodes of thelight emitting element 6113, and a capacitor element 6112 to storevoltage between a gate electrode and a source region of the TFT 6111. InFIG. 11A, the capacitor element 6112 is shown, however, in the case ofbeing capable to supply with gate capacitance of the TFT 6111 or otherparasitic capacitance, the capacitor element 6112 may not be provided.

FIG. 11B is a pixel circuit in which the pixel shown in FIG. 11A isnewly provided a TFT 6118 and a scanning line 6119. The current suppliedto the light emitting element 6113 can be forcibly stopped in accordancewith the configuration of the TFT 6118. Accordingly, a lighting periodcan be started simultaneously or shortly after a writing period beforesignals are written into all the pixels. Thus, duty ratio is increasedand moving image can be displayed specifically well.

FIG. 11C is a pixel circuit in which the pixel shown in FIG. 11B isnewly provided with a TFT 6125 and a wiring 6126. In this structure, byconnecting a gate electrode of the TFT 6125 to a wiring 6126 storing ina constant electric potential, the electric potential of the gateelectrode can be fixed, and can operate in a saturation range. Further,a gate electrode of the TFT 6111 which is operated in linear regions isserially connected to the TFT 6125, and a video signal to transmitinformation whether a pixel is lighting or non-lighting is inputted tothe gate electrode through the TFT 6110. Since voltage value between asource region and a drain region of the TFT 6111 is small, slightfluctuation of the voltage between the gate electrode and the sourceregion of the TFT 6111 does not influence on the current value which isapplied to the light emitting element 6113. Therefore, the current valueapplied to the light emitting element 6113 is determined by the TFT 6125which is operated in the saturation range. According to the lightemitting device having the above mentioned structure, brightnessunevenness of the light emitting element 6113 due to characteristicvariation of the TFT 6125 can be improved and thus the image qualitythereof can be increased. A channel length L₁ and a channel width W₁ ofthe TFT 6125, and a channel length L₂ and a channel width W₂ of the TFT6111 are preferably set to be L₁/W₁:L₂/W₂=5 to 6000:1. Further, both ofthe TFTs preferably have the same conductive type for the manufacturingsteps. For the TFT 6125, a depletion mode TFT may be used as well as anenhancement mode TFT.

Either of an analog type video signal or a digital type video signal canbe used for the light emitting device of the invention. However, in thecase of using the digital type video signal, it depends on whether thevideo signal uses voltage or current. When the light emitting element isemitted, there are a signal in constant voltage and a signal in constantcurrent, as for the video signal to be inputted to the pixel. When thevideo signal is the constant voltage, there are the one in which voltageapplied to the light emitting element is constant, and the one in whichcurrent applied to the light emitting element is constant. In addition,when the video signal is in constant current, there are the one in whichconstant voltage is applied to a light emitting element and the one inwhich constant current is applied to a light emitting element. Theformer is referred to as a constant voltage drive and the latter isreferred to as a constant current drive. In the constant current drive,constant current is applied regardless of resistance change in the lightemitting element. In the light emitting device of the invention, a videosignal of either of voltage or current for driving may be applied, andeither of constant voltage drive or constant current drive may beapplied. This embodiment can be freely combined with the aboveembodiment modes and embodiments.

Embodiment 4

In this embodiment, a transformed example of a pixel shown in FIGS. 11Ato 11C is described.

In FIG. 12A, an example of using two TFTs 6111 a and 6111 b which areserially connected instead of using a TFT 6111, in a pixel shown in FIG.11B, is shown. The TFTs 6111 a and 6111 b have the same polarity, andgate electrodes of the TFTs are connected one another. Note that thesubstitution for the TFT 6111 is not limited to the number of two, and aplurality of TFTs may be used.

Similarly in the pixel shown in FIG. 11A, a plurality of TFTs seriallyconnected can be used in substitution for the TFT 6111. Further,similarly in the pixel shown in FIG. 11C, a plurality of TFTs seriallyconnected may be used in substitution for the TFT 6125.

Next, an example of using two TFTs 6111 a and 6111 b connected inparallel in substitution for the TFT 6111 in a pixel shown in FIG. 11Bis shown in FIG. 12B. The TFTs 6111 a and 6111 b have the same polarity,and gate electrodes of the TFTs are connected one another. Note thatsubstitution for the TFTs is not limited to the number of two, and aplurality of TFTs may be used.

Similarly, in a pixel shown in FIG. 11A, a plurality of TFTs connectedin parallel may be used in substitution for the TFT 6111. Further,similarly, in a pixel shown in FIG. 11C, a plurality of TFTs connectedin parallel may be used in substitution for the TFT 6125.

In the pixels shown in FIGS. 11A and 11B, by operating the TFT 6111 in asaturation range, even when the light emitting element 6113 isdeteriorated, current value applied to both electrodes of the lightemitting element 6113 can be prevented from being decreased. Thus,brightness of the light emitting element 6113 can be prevented frombeing reduced. Further, in the pixel shown in FIG. 11C, by operating theTFT 6125 in a saturation range, even when the light emitting element6113 is deteriorated, current value applied to both electrodes of thelight emitting element 6113 can be prevented from being decreased. Thus,brightness of the light emitting element 6113 can be prevented frombeing reduced. And when the ratio of channel length to channel width ofthe TFTs 6111 and 6125 is higher, linearity of the drain current in thesaturation range can be improved. And it is desirable since reduction inthe brightness due to the deterioration can be further suppressed.However, when the channel length becomes larger, area of an island shapesemiconductor film included in the TFTs is increased, and the area ratio(antenna ratio) of the island shape semiconductor film and the gateinsulating film tends to increase. Like this embodiment, the increase ofthe antenna ratio can be controlled by using a plurality of TFTs inwhich each of the island shape semiconductor films are separated insubstitution for the TFT 6111 and the TFT 6125.

In FIG. 13, a top view of a pixel shown in FIG. 12A is shown as anexample. In FIG. 13, the TFT 6111 a and the TFT 6111 b include islandshape semiconductor films 6130 and 6131 each of which is separated.Further, the TFT 6111 b is electrically connected to a first electrode6132 included in the light emitting element 6113 through wirings 6133and 6134. Note that the wiring 6133 is formed to be in contact with thesurface of a first interlayer insulating film which covers the TFTs6110, 6118, 6111 a and 6111 b. The wiring 6134 and the first electrode6132 are formed so as to be in contact with the surface of a secondinterlayer insulating film formed over the first interlayer insulatingfilm.

Embodiment 5

In this embodiment, an appearance of a panel of a light emitting devicewhich corresponds to a mode of the present invention is described withreference to FIGS. 14A and 14B. FIG. 14A is a top view of the panel inwhich a transistor and a light emitting element formed over a substrateare sealed between the substrate and a cover material with a sealant.FIG. 14B corresponds to a cross sectional view taken along a line A-A′in FIG. 14A.

A sealant 4005 is provided so as to surround a pixel portion 4002, asignal line driver circuit 4003, a scanning line driver circuit 4004,and a protective circuit 4020 which are provided over a substrate 4001.Further, a cover material 4006 is provided over the pixel portion 4002,the signal line driver circuit 4003, and the scanning line drivercircuit 4004. Thus, the pixel portion 4002, the signal line drivercircuit 4003, and the scanning line driver circuits 4004 are sealed bythe substrate 4001, a sealant 4005 and a cover material 4006 togetherwith a filler 4007.

The pixel portion 4002, the signal line driver circuit 4003, thescanning line driver circuit 4004, and the protective circuit 4020 eachof which is provided over the substrate 4001 include a plurality ofTFTs. And in FIG. 14B, a TFT 4008 included in the signal line drivercircuit 4003, TFTs 4009 a and 4009 b included in the protective circuit4020, and a TFT 4010 included in the pixel portion 4002 are shown. TheTFTs 4009 a and 4009 b included in the protective circuit 4020 arediode-connected and serially connected by a wiring 4021.

A reference numeral 4011 corresponds to a light emitting element, andelectrically connected to the TFT 4010.

Further, a drawn wiring 4014 is a wiring for supplying a signal andpower supply voltage to the pixel portion 4002, the signal line drivercircuit 4003, the scanning line driver circuit 4004, and the protectivecircuit 4020. The drawn wiring 4014 is connected to a connectionterminal 4016 through drawn wirings 4015 a and 4015 b. The connectionterminal 4016 is electrically connected to a terminal included in an FPC4018 through an anistropic conductive film 4019.

In addition to a glass material, a metal material (typically, stainlessmaterial) and a ceramics material, a flexible material typified byplastic can be used for the substrate 4001. As the plastic material, anFRP (fiberglass-Reinforced Plastics) plate, a PVF (polyvinyl fluoride)film, a mylar film, a polyester film or an acrylic resin film may beused. Further, a sheet having a structure in which an aluminum foil issandwiched by the PVF films or the Mylar films can also be used.Furthermore, a light-transparent film such as a glass plate, a plasticplate, a polyester film, or an acrylic film is used for the covermaterial 4006.

Further, in addition to an inert gas such as nitrogen or argon, anultraviolet curable resin or a thermosetting resin may be used as thefiller 4007, and PVC (polyvinyl chloride), acrylic, polyimide, epoxyresin, silicone resin, PVB (polyvinyl butyral) or EVA (ethylene vinylacetate) can be used. In this embodiment, nitrogen is used for thefiller.

In addition, a concave portion may be provided on the surface of thesubstrate 4001 side of the cover material 4006 and a hygroscopicsubstance or a substance that can absorb oxygen is arranged therein inorder that the filler 4007 is made to be exposed to the hygroscopicsubstance (preferably, barium oxide) or the substance that can absorboxygen.

A semiconductor display device of the invention includes a panel inwhich a pixel portion with a display element is formed, and a module inwhich the panel is mounted with an IC.

Embodiment 6

A semiconductor display device of the present invention can be appliedto various kinds of electronic equipment. The following electronicequipment using the semiconductor display device of the invention isgiven as examples: a video camera, a digital camera, a goggles-typedisplay (head mount display), a navigation system, a sound reproductiondevice (such as a car audio equipment and an audio set), a notebook typepersonal computer, a game machine, a personal digital assistant (such asa mobile computer, a mobile telephone, a portable game machine, or anelectronic book), an image reproduction apparatus including a recordingmedium (more specifically, an apparatus which can reproduce a recordingmedium such as a digital versatile disc (DVD) and so fourth, andincludes a display for displaying the reproduced image), and the like.These examples of the electric equipment are shown in FIGS. 15A to 15C.

FIG. 15A is a display device, which includes a case 2001, a supportingmedium 2002, a display portion 2003, a speaker portion 2004, a videoinput terminal 2005, and the like. A semiconductor display device of theinvention can be applied to the display portion 2003. The displayportion can be thinner than that of a liquid crystal display, since thelight emitting device is a self-luminous type, and needs no back-light.Note that the light emitting display device includes all informationdisplay devices such as a personal computer, TV broadcast receive, andadvertising display.

FIG. 15B is a notebook type personal computer, which includes a mainbody 2201, a case 2202, a display portion 2203, a keyboard 2204, and anexterior connection port 2205, a mouse 2206, and the like. Asemiconductor display device of the invention can be applied to thedisplay portion 2203.

FIG. 15C is a personal digital assistant (PDA), which includes a mainbody 2101, a display portion 2102, an operation switch 2103, a modem2104, and the like. The modem 2104 may be incorporated in the main body2101. A semiconductor display device of the invention can be applied tothe display portion 2102.

As described above, the invention is widely applicable to variety kindsof electric equipment. Further, a semiconductor display device havingany structures shown in Embodiments 1 to 5 may be used for theelectronic equipment of this embodiment.

1. A semiconductor display device comprising: at least first, second andthird thin film transistors; a first interlayer insulating film formedso as to cover the first, second and third thin film transistors; first,second, third, fourth and fifth wirings formed over and within the firstinterlayer insulating film; a second interlayer insulating film formedover the first interlayer insulating film; sixth and seventh wiringsformed over and within the second interlayer insulating film; and adisplay element formed over the second interlayer insulating film andconnected to the seventh wiring, wherein the first wiring is connectedto a first terminal and a gate electrode included in the first thin filmtransistor through first and second contact holes formed in the firstinterlayer insulating film, wherein the second wiring is connected to asecond terminal included in the first thin film transistor through athird contact hole formed in the first interlayer insulating film,wherein the third wiring is connected to a first terminal and a gateelectrode included in the second thin film transistor through fourth andfifth contact holes formed in the first interlayer insulating film,wherein the fourth wiring is connected to a second terminal included inthe second thin film transistor through a sixth contact hole formed inthe first interlayer insulating film; wherein the fifth wiring isconnected to a first terminal or a second terminal included in the thirdthin film transistor through a seventh contact hole formed in the firstinterlayer insulating film; wherein the sixth wiring is connected to thesecond and the third wirings through eighth and ninth contact holesformed in the second interlayer insulating film; and wherein the seventhwiring is connected to the fifth wiring through a tenth contact holeformed in the second interlayer insulating film.
 2. A semiconductordisplay device according to claim 1, wherein the display element is alight emitting element, the light emitting element comprising: a firstelectrode; an electroluminescent layer formed over the first electrode;and a second electrode formed over the electroluminescent layer, whereinthe second electrode has a light-transparent property.
 3. Asemiconductor display device according to claim 1, wherein the secondinterlayer insulating film is formed by a coating method.
 4. Asemiconductor display device according to claim 1, wherein the secondinterlayer insulating film is formed of a plurality of insulating filmsand any one of the plurality of insulating films is formed by a coatingmethod.
 5. A semiconductor display device according to claim 1, whereinthe second interlayer insulating film comprises an organic resin film oran inorganic insulating film.
 6. A semiconductor display deviceaccording to claim 1, wherein the second interlayer insulating filmcomprises an insulating film formed by using a material of a siloxanegroup.
 7. A semiconductor display device comprising: a display element;a first thin film transistor for electrically connected to the displayelement; a driver circuit electrically connected to a signal line; and aprotective circuit electrically connected to the signal line, theprotective circuit comprising: second and third thin film transistors; afirst interlayer insulating film formed over the second and third thinfilm transistors; first, second, third and fourth wirings formed overand within the first interlayer insulating film; a second interlayerinsulating film formed over the first interlayer insulating film, thefirst wiring, the second wiring, the third wiring and the fourth wiring;and a fifth wiring formed over and within the second interlayerinsulating film, wherein the first wiring is electrically connected to afirst terminal and a gate electrode included in the second thin filmtransistor; wherein the second wiring is electrically connected to asecond terminal included in the second thin film transistor; wherein thethird wiring is electrically connected to a first terminal and a gateelectrode included in the third thin film transistor; wherein the fourthwiring is electrically connected to a second terminal included in thethird thin film transistor, and wherein the fifth wiring is electricallyconnected to the second wiring and the third wiring.
 8. A semiconductordisplay device according to claim 1, wherein the first and second thinfilm transistors are provided in a protective circuit and the third thinfilm transistor is provided in a pixel portion.
 9. A semiconductordisplay device according to claim 8, wherein the protective circuit isprovided between the pixel portion and a driver circuit.
 10. Asemiconductor display device according to claim 7, wherein theprotective circuit is provided between the signal line and the drivercircuit.
 11. A semiconductor display device according to claim 8,wherein the protective circuit is connected to a signal line.
 12. Anelectronic equipment having the semiconductor display device accordingto claim 7, wherein the electronic equipment is selected from the groupconsisting of a video camera, a digital camera, a goggle type display, anavigation system, a sound reproduction device, a notebook type personalcomputer, a game machine, a personal digital assistant, and an imagereproduction apparatus including a recording medium.
 13. A semiconductordisplay device according to claim 8, wherein the protective circuit isconnected to a scanning line.
 14. A semiconductor display deviceaccording to claim 7, wherein the second interlayer insulating film isformed by a coating method.
 15. A semiconductor display device accordingto claim 1, wherein the first interlayer insulating film comprises asilicon oxide film, a silicon nitride film, or a silicon oxynitridefilm.
 16. A semiconductor display device according to claim 7, whereinthe second interlayer insulating film is formed of a plurality ofinsulating films and any one of the plurality of insulating films isformed by a coating method.
 17. A semiconductor display device accordingto claim 7, wherein the second interlayer insulating film comprises anorganic resin film or an inorganic insulating film.
 18. A semiconductordisplay device according to claim 7, wherein the first interlayerinsulating film comprises a silicon oxide film, a silicon nitride film,or a silicon oxynitride film.
 19. A semiconductor display deviceaccording to claim 7, wherein the second interlayer insulating filmcomprises an insulating film formed by using a material of a siloxanegroup.
 20. A semiconductor display device according to claim 1, whereinthe gate electrode included in the first thin film transistor, the gateelectrode included in the second thin film transistor and a gateelectrode included in the third thin film transistor are taper-shaped.21. A semiconductor display device according to claim 7, wherein a gateelectrode included in the first thin film transistor, the gate electrodeincluded in the second thin film transistor and the gate electrodeincluded in the third thin film transistor are taper-shaped.
 22. Asemiconductor display device according to claim 1, further comprising: abarrier formed over the second interlayer insulating film.
 23. Anelectronic equipment having the semiconductor display device accordingto claim 1, wherein the electronic equipment is selected from the groupconsisting of a video camera, a digital camera, a goggle type display, anavigation system, a sound reproduction device, a notebook type personalcomputer, a game machine, a personal digital assistant, and an imagereproduction apparatus including a recording medium.
 24. A semiconductordisplay device according to claim 7, further comprising: a barrierformed over the second interlayer insulating film.